This research will investigate, at the level of an identified neuron, extrinsic and intrinsic factors that regulate neuronal phenotype in a steroid-sensitive system. In the hawkmoth, Manduca sexta, steroid hormones (ecdysteroids) orchestrate the postembryonic reorganization of the nervous system during metamorphosis. Steroid hormones likewise regulate neurodevelopmental events in vertebrates. Insect and vertebrate steroid receptors belong to the same superfamily of DNA-binding transcription factors, suggesting that molecular genetic mechanisms may be shared. Identification of fundamental mechanisms by which steroid hormones and other factors influence neuronal phenotype should assist in developing clinical strategies to prevent or treat birth defects and neurological disorders in humans, especially those involving endocrine malfunctions. The proposed experiments will focus on a larval motor neuron in Manduca, designated APR. APR homologs occur in each abdominal segment, where they innervate a proleg retractor muscle. An elevation of ecdysteroids at pupation causes APR's dendrites to regress dramatically, followed by the programmed death of APRs in a subset of segments. APR's target muscle also degenerates in a segment-specific pattern, but the pattern does not match that of the APRs. APRs that survive pupation are respecified for new functions in the pupal stage. This unique segment-specific pattern of APR fates provides an exceptional opportunity to investigate how extrinsic and intrinsic factors cause segmentally homologous neurons to express different phenotypes in response to the same hormonal cue. SPECIFIC AIM 1: To test how extrinsic factors (interactions with muscles, sensory neurons and interneurons) and intrinsic factors (ecdysteroid receptors; EcRs) contribute to the segment-specific fates of APRs in vivo. We will investigate the effects of manipulating the APRs' interactions with target muscles and afferent inputs, and determine whether developmental changes in interneuronal inputs to APRs vary by segment. The segmental pattern of EcR expression in APRs will also be determined. The working hypothesis is that the segment- specific pattern of APR death results from the activation, via EcRs, of intrinsic developmental programs that do not depend on interactions with other cells. SPECIFIC AIM 2: To use cell culture to test the hypothesis that dendritic regression and segment- specific death of APRs results from the activation via EcRs, of intrinsic developmental programs that do not depend on interactions with other cells. Fluorescently labelled APRs will be placed in low density cell culture and treated with physiological levels of ecdysteroids. EcR expression in cultured APRs will also be determined. If ecdysteroids evoke regression and/or death, we will begin to investigate the underlying mechanisms.